ENGINE BALANCE: Inline 3 vs. Inline 4 vs. Inline 5 vs. Inline 6

The engine block that you see has a stroke of 77mm, which means that the piston covers 77mm of travel during the full length of it's up and down motion. By this logic, rotating the crankshaft 90 degrees from top dead center should set the piston at half the stroke, in our example the piston should cover exactly 38.5 mm. Yet, as you can clearly is here it doesn't'. During the first 90 degrees of the crankshafts travel the piston covers more than half the stroke.
So why is this happening? It's happening because the piston is connected to the crankshaft by a connecting rod and the connecting rod doesn't just go up and down in a simple reciprocating motion. Instead the connecting rod steps out from a linear path of travel both left and right.

What is primary engine balance? The source of primary balance or imbalance is simply the mass of the reciprocating parts of your enigine, which means that your pistons are the greatest potential source of a primary engine imbalance. An engine with an odd number of pistons can have a primary imbalance if the reciprocating mass of the odd piston isn't properly canceled out by the other pistons.

We're starting with one that has the least number of cylinders the inline three engine. When it comes to inline three engines most of them have their crank throws 120 degrees angled away from each other. Such an angle distributes the crank throws evenly across the crankshaft which then enables an even distribution of cylinder firings.

The inline three engine has good primary balance due to the even spacing of it's crank throws. If you look at the crank from the front this becomes even more apparent. What about the secondary balance? Wel l that's actually pretty good too, because the three pistons are always in different parts of the upper and lower half of the cranks rotation and no two pistons move together. This sort of disperses the negative effects of the unequal piston speeds in the different halves of the cranks rotation.
But if your gut feeling is telling you there has to be a problem with an odd number of cylinders, you were right. And the issue is discovered by drawing a line across the middle of the inline three cylinder. Notice anything weird? The force on this side is obviously unequal to the force on this side. the engine's center of gravity is at the middle of cylinder two and the unequal forces at different sides of the center of gravity mean that the engine rocks back and forth or end to end.

So how do we fix the unbalance? Well the solution to fixing an unbalance in any kind of reciprocating piston engine can come in the form of a balancing shaft. In the case of the inline three we need a single balance shaft with weights which moves in the direction opposite to the piston travel to balance out the the end to end rocking of the inline three.

But this doesn't mean that all inline three engines have a balancing shaft. A balancing shaft adds cost, weight and friction which is why most manufacturers will try to avoid it whenever possible. Ford's 1.0 liter three cylinder ecoboost engine is an example of a pretty smooth inline three cylinder that has no balancing shaft. Instead the engine uses an unbalanced flywheel and crank pulley and highly engineered engine mounts to cancel out most of the front to back rocking.

Now let's add one more cylinder to the mix and talk about the inline four cylinder engine.
The inline four also has perfect primary balance as you can see each upward motion of a piston is canceled out by the downward motion of another piston. When two go up, two pistons go down.
As you can see when two piston are at the top two pistons are at the bottom this means that the secondary balance forces associated with unequal speeds at the top and bottom part of the cranks rotation aren't just present in the inline four they are in fact augmented by the fact that the pistons move in pairs.

The easiest way to understand the balance associated with the inline 5 engine is to think of it as the inline three's big brother. They share the same balance genetics, with the inline 5 being the larger more powerful version.

What about he inline six? Well, it's an inline three standing in front of a mirror. It's that simple. Unlike any of our previous configurations the inline six has a perfect primary and secondary balance. We have an even number of pistons and no two pistons occupy the same position of the stroke at one time.

A special thank you to my patrons:
Daniel
Peter Della Flora
Daniel Morgan
William
Richard
Pepe
Brian Durning
Andrew Ruud

00:00 Secondary balance
03:00 Primary balance
04:19 Inline 3
10:08 Inline 4
13:49 Inline 5
16:26 Inline 6

#d4a #enginebootcamp #enginebalance

[owenbassist]

This guy explains better than my mechanics teacher

[TopiasSalakka]

Can't use an inline 6 in a transverse FWD application?
Volvo: hold my beer

[lazaros]

Please do yamahas crossplane, hearing so much about how smooth it is, how the rider feels so connected with the throttle and how it doesn't stress the tyres. Thank you!

[Profect]

Oh and i was having a bad time understanding secondary balances this last week and then you show up, you've read my mind
Thank you for sharing such amazing content

[SC]

This is a fun topic, and I learned some new things. The longitudinal rocking is new to me, but it reminds me of issues that aircraft engine manufacturers encountered in the late 1930s with double row radial engines. On a big radial, one cylinder has a (sort of) conventional connecting rod that is connected to the crank pin. The rest are connected to that with pins that are not concentric to the crank pin. So when the rod makes the pendulum motion that you described, the pistons, with their “link” rods tend to wrap slightly, causing them all to undergo different acceleration rates throughout the stroke of the main rod. The result is a small circular motion of the entire engine, where the cylinder centerlines remain on their static plane. If the engine has seven cylinders, the motion is nearly negligible, but with nine it’s a bit more severe. For single row engine, it’s not a problem, but if it’s a double row (eighteen cylinders), big problem! With that configuration, the two crank pins are 180° apart, like bicycle pedals. The circular motion, phased 180° apart, resulted in a wobbling motion of the whole engine, that attempted to constantly change the plane of rotation of the propeller. Watch out prop shaft and main bearings! The fix was to employ two sets of second order balances, rotating at twice crankshaft speed. This is what it took Pratt and Whitney and Wright to cross the 2, 000 HP threshold, just in time for WWII Corsairs, Hellcats and others.

[jozsefizsak]

It's so great that you're doing this. While I always enjoyed the humour that you often added to your dissertations, I'm not complaining because I so appreciate your efforts that I'm content to have these in whatever style you most enjoy producing. Thank you so much!

[cipher]

One of the most UNDERRATED channels I've ever seen
I'm seriously looking forward to this balance series man
Love your work
BTW, if it isn't too much too ask, can you also cover different firing orders please?
Thanx a lot D4A <3

[ThirtyOught]

Dang, I love this guy's videos! Clear, concise, to-the-point, no long-winded irrelevant info. I'm dying to hear you explain & evaluate Freddie Spencer's 1983 Honda NS-500 V3 TWO-stroke Gran Prix motorcycle race bike! And maybe also Bugatti's W-16.

[trevorcalhoun]

The straight eight is another well balanced engine and one I have always liked.

[RayMorgenstern]

Every time someone sees my i3 Polo engine doing it's Macarena thing in the bay, it seems weird to them. I'll just reference this video from now on. Great video!

[epiploon]

It’s a 6 cyl video! Perfectly balanced!

[Mrgoodnyt]

Excellent video mate very clear and easy to understand! looking forward to more. This channel is so underrated.

[jackdarcy]

can't wait for a vid on V configurations. curious to see if V8 dampens secondary imbalance in inline 4, and to hear how a V12 is just two inline 6s duct taped together.

[imShivamKumar]

Holy moly, this is the best video on this topic on whole of youtube, never stop making these videos you guys, this is amazing.

[rocketmantm]

If you are going to make a series on engine configurations, please include radial engines. Also thank you for making these videos, I appreciate the work you put into these.

[gtspeed]

This is well-presented example on primary and secondary engine balance. The preface showing piston speed differences during crank shaft rotation was monumental in explaining discrepancies of the latter balance. Nicely done!

[olavcramer]

The best way to explain engine primary and secondary nuances yet. Thanks so much for making these! Now I'm really looking forward to the next Mazda straight 6. Let's hope it's not just DI and also throws in some occasional intake valve cleaning Port Injection for a truly remarkable engine.

[SPimentaTV]

This is the best YouTube channel about car mechanics theory!

[lookronjon]

My brother had a Suzuki sprint in the Virgin Islands and had a three cylinder turbo and it actually screamed. My Toyota Land Cruiser has a straight six and it runs beautifully. Thanks for explaining this for us motorheads.

[bicyclist]

The in line 6 was used in the early Jag E-Types. The in line 5 was used in the Volvo 7 and 8 series sedans and estates as well as VW Golf GTi's. The Lotus Esprit used a in line four for decades. You don't see very many in line 3's in the US. This was very educational. I wish I'd learned about this when I was in school. Thank you.